Cellulose esters are valuable polymers which are useful in many plastic, film, coating and fiber applications. In coatings applications, cellulose esters are generally applied from organic solvent solutions. Because of health and environmental concerns from volatile organic content (VOC) of solvent based coatings, water based coatings are becoming more in demand. There are waterborne coating formulation available containing polyesters and acrylics, but few contain cellulose esters due to their hydrophobic nature making them difficult to disperse or emulsify. When applied from organic solvent solutions as wood, leather, and metal coatings, cellulose esters impart desired properties such as, improved pigment dispersions, improved metallic flake orientation, improved solvent release, and higher gloss coatings. Cellulose esters also function as an excellent rheology control agent in solvent coating formulations. It would be very desirable to incorporate these excellent properties of cellulose esters into waterborne coating systems.
Previous attempts to produce a water dispersible cellulose ester coatings system have involved oxidation of cellulose ester, grafting of acrylic acid to cellulose esters, and reaction of anhydrides such as phthalic anhydride or succinic anhydride with the hydrolyzed cellulose esters to provide modified cellulose esters containing free carboxyl groups such as disclosed in the following U.S. Pat. Nos. 4,435,531; 4,590,265; 4,960,814; 4,252,697. However, these processes-are complicated resulting in expensive aqueous dispersion. It would, therefore, be very desirable to be able to easily produce aqueous dispersions containing unmodified cellulose esters.
Other cellulose ester systems used in water based coating are disclosed in U.S. Pat. No. 3,318,714 (describing pigment dispersions in a cellulose acetate butyrate lacquer systems) and U.S. Pat. No. 3,220,865 (describing an emulsion coating composition based on cellulose acetate butyrate, and plasticizers).
In addition to low VOC, stability of the dispersion is an important feature for these aqueous dispersions to be useful in coatings applications. Many aqueous dispersions, including those of low VOC, are not able to be stored for long periods of time without constant stirring. It would, therefore, be very desirable to be able to produce stable aqueous dispersions of cellulose esters, particularly ones that are stable over long periods of time. It is even more desirable to be able to produce stable aqueous dispersions that are low in VOC.